Several different processes are used to recover tungsten from minerals such as wolframite and scheelite. P. Borchers presented a review article entitled "Processing of Tungsten" (Published on page 64 of a book entitled "Tungsten". Proceedings of the First International Tungsten Symposium, Stockholm, 1979). Borchers indicated that it was common practice to dissolve the tungsten contained in various types of minerals in either acidic media (usually aqueous hydrochloric acid) or alkaline media (i.e. aqueous solutions containing stoichiometrically excess amounts of sodium carbonate and/or sodium hydroxide). It is reported in the literature, and it is known to those skilled in the art, that high grade scheelite-type concentrates are frequently processed using the aqueous hydrochloric acid route; wolframite is often processed using sodium hydroxide, and lower grades of scheelite-type concentrates are often processed using a pressure leaching route utilising aqueous sodium carbonate (with the possible addition of some sodium hydroxide). Processes for the purification and processing of such acidic and alkaline leach liquors to recover various tungsten-containing compounds are well described in the literature. In the alkaline routes the tungsten is frequently present in the form of dissolved sodium tungstate species which are present in aqueous solutions which may also contain sodium hydroxide and/or sodium carbonate. It is usual to neutralise chemically the sodium hydroxide and/or sodium carbonate using sulphuric acid, prior to the conversion of the sodium tungstate species to ammonium tungstate as an intermediate in the production of ammonium paratungstate crystals. To those skilled in the art it is known that even in the hydrochloric acid leaching processes, purification steps can exist in which the tungsten appears as dissolved sodium tungstate species, and that such species then need to be converted to ammonium tungstate prior to conversion to ammonium paratungstate crystals.
One of the methods for converting sodium tungstate to ammonium tungstate which has been accepted by industry, involves the use of a liquid solvent extraction process. T. M. Kim and M. B. MacInnis describe such a process. (Extractive Metallurgy of Refractory Metals. Edited by H. Y. Sohn et al. Proceedings of a symposium sponsored by the TMS-AIME Refractory Metals Committee and Physical Chemistry of Extractive Metallurgy Committee at the 110th AIME Annual Meeting, Chicago, Ill. 1981). During such a solvent extraction process the sodium associated with the tungstate anion is converted to sodium sulphate. Hence it is observed that existing processing technology results in the conversion of virtually all the sodium ions associated with sodium hydroxide, carbonate or tungstate to the soluble sodium sulphate form. Y. A. Topkaya and H. Eric (in a paper entitled "Laboratory Testing of Uludag Scheelite Concentrate for the Production of Ammonium Tungstate" presented at the MINTEK 50 International Conference on Recent Advances in Mineral Science and Technology at Sandton in South Africa during 1984) described the testing of a processing route which utilises some of the features indicated above, and demonstrates how all the sodium carbonate initially introduced to the process for leaching of the tungsten reports finally as sodium sulphate.
The existing tungsten extraction technology which produces aqueous effluents containing sodium sulphate and other types of contaminants such as calcium chloride and/or sodium chloride thus exhibit disadvantages as follows:
(a) The acid required to neutralise the sodium hydroxide, carbonate or tungstate effectively can represent a substantial cost, and the acid is not recovered or reused.
(b) The original source of the sodium ions in the process (i.e. sodium hydroxide or sodium carbonate) can also represent a substantial cost, and such bases are also not recovered or reused.
(c) The neutral dissolved salts which are ultimately produced represent pollutants. Environmental legislations for the disposal of aqueous effluents are generally becoming more stringent, and the cheap disposal of effluents containing significant concentrations of such dissolved salts is generally not possible.
(d) The water associated with substantial concentrations of dissolved salts cannot be recycled, and needs to be disposed of. This represents another cost, and a waste of water.
British Patent Specification No. 2 137 658 describes a method whereby sodium cations are removed from an anolyte through a cation selective membrane into a catholyte under the influence of an electrical potential. This method is used in the context of concentrating dilute caustic alkali and of increasing the life of the electrodes by periodic current reversal.
British Patent Specification No. 2 073 780 teaches the purification of molybdenum compounds by moving cations through a cation selective membrane in an electrolytic cell. It is to be noted, however, that the molybdenum compounds in this process are in essentially insoluble form.